I see this type of inference made a lot, for example in the context of distributional justice: A makes an investment. A thinks this investment was a rational decision, until he learns that B also profited from A making this investment. A now no longer thinks the investment was rational, and that some measure must be taken to prevent these "undeserved profits" on B's part from occurring — even when B is not seen as an enemy, but the general attitude towards B is neutral.

Since B is not an enemy whose benefit is seen as a problem in itself, and the act was seen as rational in itself, it is clearly a fallacy to infer from the new evidence of B's benefit that the act is no longer rational (or even weaker: that there is some ethical problem with this situation). Is there a widely used name for this?


Thanks to the comments, and after considering some examples, I have concluded that my question is indeed describing a situation that doesn't really occur. In those situations that I was thinking of, an action is criticized not for being irrational but for being unjust, despite leaving everyone better off than before. While I find this intuitively odd, it is not a fallacy.

  • I'm not sure we have enough details to tell if this is actually a fallacious inference. For instance, if B is known to make bad decisions and you realize you acted as B did, then that is some evidence that your decision is a bad one.
    – E...
    Mar 10, 2019 at 19:04
  • the inference i am interested in makes no further assumptions than the ones I mentioned. the action is deemed bad only because it has some unintended side effect of benefiting someone else who "did nothing to earn" this benefit. Mar 10, 2019 at 20:16
  • The only formal problem in this scenario I see is the free-rider problem. Here B is a free-rider (aka parasite), and indeed this legitimately can effect the rationality of the investment, as it can lower profits, or longer term even make a profitable investment into an unprofitable one. However B is benefiting from but not contributing to this arrangement, that money has to come from somewhere, and it’s either A’s pocket directly, or the returns which otherwise would be reinvested and improve the system, etc.
    – Dan Bron
    Mar 11, 2019 at 11:51
  • This involves ethical values, and is not obviously fallacious. The assessment of one's status in a society is always relative, so learning that B gains a benefit of the same sort as A (be it money or a distinction) does legitimately devalue it for A. To a degree, this is associated with values such as justice (beyond that it becomes envy, an extreme case is described in Aesop's fable Greed and Envy). This may not mean that the decision was outright "irrational", but it certainly alters the utility calculation significantly.
    – Conifold
    Mar 11, 2019 at 19:02
  • There is a mistake of some sort being committed here, which concerns the inference from the new availability of information to the irrationality of the previous decision. This could be viewed as an instance of what Wikipedia names a Historical Fallacy (attributing it to Dewey).
    – Paul Ross
    Mar 16, 2019 at 13:40


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